PRIMARY fixatives: COAGULANTS IO3 



membered that there is more shrinkage than this after the action of 

 most fixatives (see fig. lo, p. 79). 



Hardening. Mercuric chloride hardens moderately. Wetzel's 

 figure is about iioo, nearly 5 times that for chromium trioxide. 

 There is considerable further hardening on subsequent soaking in 

 80% ethanol. 



Immediate effects on particular constituents of the cell. The shape 

 of the cell, including any pseudopodia, is rather well preserved. 

 The cytoplasm is sometimes much more finely coagulated than 

 one would expect of a fixative of this sort. Indeed, mercuric 

 chloride is particularly recommended by Policard and his col- 

 leagues *^* on this account. Cytoplasmic inclusions of various 

 sorts, including mitochondria and the neutrophil granules of poly- 

 morphs, are preserved, though lipid globules may fuse. The 

 nuclear membrane is clearly seen; nuclear sap is finely coagulated, 

 the nucleolus very distinct. The mitotic spindle appears fibrous. 

 Mercuric chloride distorts the cell less than any other coagulant 

 fixative. 



Methods of washing out. Since mercuric chloride is readily 

 soluble in ethanol, there is no purpose in washing in water if the 

 tissue is going to be dehydrated. 



Mercuric chloride has a troublesome tendency to produce ex- 

 trinsic artifacts. These appear in the final preparation as small, 

 black, amorphous particles, down to about i^ in diameter, and 

 also needle-shaped, birefringent crystals, up to 25/x long, usually 

 with a lump of the black material at each end. Both artifacts have a 

 tendency to be deposited eventually on the surfaces of the slide 

 and coverslip, in contact with the tissue. 



Mayer ^^^ concluded that the black material was metallic mer- 

 cury, but he could not determine the chemical composition of the 

 crystals. It is unlikely that they consist of calomel, since they are 

 also seen after the use of Millon's reagent,^*^ which contains no 

 chloride. Mayer was unable to produce such crystals from mercuric 

 chloride or Millon's reagent except by their action on the tissues of 

 organisms. The crystals, unlike the black material, are not present 

 as such until the tissue is brought into a mounting medium, such 

 as Canada balsam; cedarwood oil is particularly apt to produce 

 them. 



The black artifact is removed by the action of iodine in alcoholic 

 solution. This was discovered in 1886.^-^'^^^ Presumably the 

 mercury is oxidized to mercuric iodide, which is soluble in 



